iplink: Improve index parameter handling

iplink: Improve index parameter handling

Correctly check for valid network device index supplied on
command line: indexes are always greather than zero. Check
for duplicate "index" argument.

Initialize @index to 0 to simplify handling it in iplink_modify().
Other callers (link_veth.c, iplink_vxcan.c) already did so.

No need to initialize ifi_index with 0 since it is already
initialized at the @struct req initialization time and not
modified in iplink_parse().

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>